Plastic-lined pipe offers advantages over standard metallic pipe in certain applications. For example, a pipe lined with an inert plastic can be used to carry a liquid which is corrosive to the outer metal pipe. Plastic-lined pipe lengthens the life of a sprinkler system because of the rust free interior. Additionally, plastic-lined pipe can provide greater flow rate because frictional loss is reduced due to a higher Hazen-Williams pipe roughness coefficient or C-factor.
Various methods have been proposed for manufacturing plastic-lined metallic pipe. In one such method, a plastic sleeve of polyethylene is inserted into an outer aluminum tube that is being seam-welded to produce lined aluminum tubing which is wound into a coil. The plastic sleeve is folded under a shoe, extending downstream of the welder, which flattens it against the underside of the aluminum tubing, keeping it spaced apart from the heat of the seam-welding operation. Gas under high pressure is used to inflate the flattened sleeve to its full diameter and force it against the interior surface of the welded tubing. The welded tubing can be swaged to a smaller diameter following the inflation of the interior sleeve. This process is not continuous, as a coil of plastic sleeve of some finite length is employed, which allows air to be exhausted from the end of it. For additional information concerning the structure and operation of the apparatus employed in this process, reference may be made to U.S. Pat. No. 3,069,763.
In another prior art method of making plastic-lined metal pipe, a steel pipe of finite length is lined with a plastic sleeve by first stretching the sleeve longitudinally. After the stretched sleeve is inserted into the steel pipe, it is released to return, due to its resiliency, toward its as-formed diameter thereby engaging the inner surface of the pipe. For additional information on this method, reference may be made to European Patent Application No. 0226410.